The primary goal of this study is to investigate the asymmetric structure of continental shelf wave in a semienclosed double‐shelf basin, such as the Yellow Sea. Supported by in situ observations and realistic numerical simulations, it is found that in the Yellow Sea, the shelf wave response to the synoptic wind forcing does not match the mathematically symmetric solution of classic double‐shelf wave theory, but rather exhibits a westward shift. To study the formation mechanism of this asymmetric structure, an idealized model was used and two sets of experiments were conducted. The results confirm that the asymmetric structure is due to the existence of a topographic waveguide connecting both shelves. For a semienclosed basin, such as the Yellow Sea, a connection at the end of the basin eliminates the potential vorticity barrier between the two shelves and hence plays a role as a connecting waveguide for shelf waves. This waveguide enables the shelf wave to propagate from one shelf to the other shelf and produces the asymmetric response in sea level and upwind flow evolutions.

The seasonal occurrence of hypoxia in the bottom waters of the northern Gulf of Mexico has been monitored for over 30 years (Rabalais et al., 2007; Rabalais et al., 2001). In general, the processes regulating the spatial and temporal variability of the hypoxic area in the region are attributed to variations in the nutrient load and the extension of the freshwater envelope generated by the Mississippi-Atchafalaya River system (Bianchi et al., 2010; Rowe & Chapman, 2002; Scavia et al., 2003). However, most mechanistic studies in the regional literature are concerned with the relationship between these two drivers and the full extent of hypoxia, while the internal variability and short-term shifts have been insufficiently investigated. Furthermore, the severity of hypoxia has often been equated to its areal extent, such that most modeling efforts are directed to estimate this quantity with accuracy. Official estimates of extent for managerial

This study describes a specific type of critical layer for near-inertial waves (NIWs) that forms when isopycnals run parallel to sloping bathymetry. Upon entering this slantwise critical layer, the group velocity of the waves decreases to zero and the NIWs become trapped and amplified, which can enhance mixing. A realistic simulation of anticyclonic eddies on the Texas-Louisiana shelf reveals that such critical layers can form where the eddies impinge onto the sloping bottom. Velocity shear bands in the simulation indicate that windforced NIWs are radiated downward from the surface in the eddies, bend upward near the bottom, and enter critical layers over the continental shelf, resulting in inertially-modulated enhanced mixing. Idealized simulations designed to capture this flow reproduce the wave propagation and enhanced mixing. The link between the enhanced mixing and wave trapping in the slantwise critical layer is made using ray-tracing and an analysis of the waves’ energetics in the idealized simulations. An ensemble of simulations is performed spanning the relevant parameter space that demonstrates that the strength of the mixing is correlated with the degree to which NIWs are trapped in the critical layers. While the application here is for a shallow coastal setting, the mechanisms could be active in the open ocean as well where isopycnals align with bathymetry.

Over the Texas-Louisiana Shelf in the Northern Gulf of Mexico, the eutrophic, fresh Mississippi/Atchafalaya river plume isolates saltier waters below, supporting the formation of bottom hypoxia in summer. The plume also generates strong density fronts, features of the circulation that are known pathways for the exchange of water between the ocean surface and the deep. Using high-resolution ocean observations and numerical simulations, we demonstrate how the summer land-sea breeze generates rapid vertical exchange at the plume fronts. We show that the interaction between the land-sea breeze and the fronts leads to convergence/divergence in the surface mixed layer, which further facilitates a slantwise circulation that subducts surface water along isopycnals into the interior and upwells bottom waters to the surface. This process causes significant vertical displacements of water parcels and creates a ventilation pathway for the bottom water in the northern Gulf. The ventilation of bottom water can bypass the stratification barrier associated with the Mississippi/Atchafalaya river plume and might impact the dynamics of the region’s dead zone.

Wind is a primary forcing agent for river plume variability. Consequently, the temporal resolution of wind forcing is an important factor to consider for river plume simulations. This study evaluates river plume simulation errors caused by temporal subsampling of wind forcing data. We use an idealized model of a river plume over a continental shelf and force the model with temporally filtered winds to quantify the effect of temporal subsampling on simulation accuracy. The simulation error is proportional to the fraction of energy missing in the high‐frequency wind absent from the forcing. These results set requirements for temporal wind resolution in realistic simulations of river plumes. Spectral analysis of observed wind records at the Mississippi River, Columbia River, and Merrimack River regions indicates that, for simulation errors due to insufficient temporal wind resolution to be smaller than 5% of the variance, 3‐hourly or 4‐hourly wind data are reasonable. Though horizontal variations in wind forcing are lost, analyzing fast Fourier transformation spectrum of a single‐point wind measurement in the simulation region is helpful for estimating simulation errors due to temporal resolution and hence aid in properly selecting temporal resolutions.

Abstract. Offline advection schemes allow for low-computational-cost simulations using existing model output. This study presents the approach and assessment for passive offline tracer advection within the Regional Ocean Modeling System (ROMS). An advantage of running the code within ROMS itself is consistency in the numerics on- and offline. We find that the offline tracer model is robust: after about 14 d of simulation (almost 60 units of time normalized by the advection timescale), the skill score comparing offline output to the online simulation using the TS_U3HADVECTION and TS_C4VADVECTION (third-order upstream horizontal advection and fourth-order centered vertical advection) tracer advection schemes is 99.6 % accurate for an offline time step 20 times larger than the online time step as well as online output saved with a period below the advection timescale. For the MPDATA tracer advection scheme, accuracy is more variable with the offline time step and forcing input frequency choices, but it is still over 99 % for many reasonable choices. Both schemes are conservative. Important factors for maintaining high offline accuracy are outputting from the online simulation often enough to resolve the advection timescale, forcing offline using realistic vertical salinity diffusivity values from the online simulation, and using double precision to save results.

Tracer variance budgets can be used to estimate bulk mixing in a control volume. For example, simple, analytical, bulk formulations of salt mixing, defined here as the destruction of salinity variance, can be found for estuaries with a riverine source of fresh water and a 2-layer exchange flow at the mouth using salinity as a representative tracer. For a steady case, the bulk salt mixing, M, can be calculated as M = QRS2out + Qin (Sout − Sin)2, where Sin and Sout are the representative salinities in the estuarine exchange flow, and QR and Qin are the river and landward volume fluxes, respectively. M can be considered as the sum of mixing pathways, where each pathway has a mixing of Q (ΔS)2, where Q is the volume transport and ΔS is the salinity difference across the pathway. For the estuary case, one mixing path is associated with the river inflow, the other is associated with the inflow of salty, oceanic water. This concept of linking mixing to input-output pathways is extended, in simple box models, from estuaries to scenarios with multiple inputs/outputs, as might be found in a complex estuarine/fjord network, in a region on a continental shelf, or any other control volume with multiple exchanges. This approach allows for the estimation of the relative contributions of each input-output pathway to the total mixing within a control volume.

Enhanced mixing over sloping bathymetry plays an important role in closing global ocean energy budgets and influences deep-water mass transformation and diapycnal upwelling, yielding significant implications for salinity, heat, and nutrient budgets in the abyssal ocean (